From Jim Cosgrove's Octopus Adventures:

--when the octopus is not hunting or mating it mainly stays inside its den.
--when there is activity going on outside the den, the octopus will stay inside and "no human is strong enough to pull it out."
--the suckers and the inside of the mantle (both of which are white) are the only areas that cannot change colors.
--when need be they will hold their breath so the white inside of the mantle won't show, but they can do this for no longer than 5 minutes.

From the article on the Giant Pacific Octopus:

--it's a mollusc in the same family as snails, clams, and oysters.
--more specifically it falls under the classification of cephalopod meaning "head-foot" because its 8 "feet" (arms) are directly attached to its head around the mouth/beak.
--some variety of octopus makes an appearance in all the world's oceans, but they don't live in fresh water.
--the Giant Pacific Octopus is the largest octopus in the world.
--females usually don't exceed 55 lbs.
--it tastes with its suckers, of which there are 2 rows per arm-up to 1600 in all.
--they are invertebrates--a.k.a. no bones.
--the only hard part of their body is the beak which is made out of a material similar to human fingernails.
--their bite is poisonous, although the only octopus that is known to have killed a human with its bite is the Blue-ringed Octopus of Australia, usually the venom just paralyzes the victim temporarily.
--octopuses have highly developed brains and fantastic eyesight.
--the body of an octopus is called the mantle-it looks like a bag and moves with the breath (like 3-D breath in movement only much more so).
--the mantle holds the stomach and all the other organs including 3 hearts- 1 at the end of each of the 2 gills and another pumping blood through the rest of the body.
--their blood is pale blue (kind of reminiscent of the aliens in Galaxy Quest!).
--RESPIRATION:
1-octopus breathes in
2-water flows over gills and fills mantle
3-breath goes out and water is forced through a tube (siphon)
--if the octopus is trying to escape it can force water through the siphon at increased speed and propel itself backwards.
--ink is also used as a diversionary tactic when a predator comes too close for comfort.
--females live 3.5 years on average.
--females lay 1000s of eggs-as many as 50,000+ tending them for about 6 months during which time she doesn't eat.
--when the babies first hatch they are approximately the size of a grain of rice.
--it may take up to 3 years for them to grow to the size of their parents.